For some years, beginning prior to World War II and with increasing volume, the metal cutting industry has been using toolholders with hard metal inserts to contact the work. Early on the hard metal, in the form of tungsten carbide, was brazed onto a steel shank. Many different grades of carbide have been developed. One departure from the brazed-on tip was the use of a mechanical toolholder which would hold a slug type of insert, that is, an insert square, triangular or round in cross-section in a range of 1/2" to 3/4" in thickness and having a length of 11/2 to 2" or more. This type of insert was held more or less vertically with proper clearance rake angles at the front of a mechanical toolholder and was intended to be backed by a back-up screw and fed upwardly by that screw as the insert was worn and reground to sharpness.
The next phase of development produced the so-called throwaway inserts (TA) which were in the form of small flat pellets of tungsten carbide. Toolholders were fashioned with a small pocket at one corner with a flat support wall and back-up side walls leaving a corner open to expose a retained insert to the work. Various top clamps and pin-type retaining devices were utilized to secure the small inserts securely in the holders.
As a refinement on these inserts, so-called chip control grooves were ground into the top surfaces spaced inwardly from the cutting edge to provide chip control of the cut metal. This involved curling the removed metal in a way to avoid long entangled strips of metal. Preferably, the chips were curled in the form of a helix and broken off in lengths which would drop away from the work. Later these chips control grooves were pressed into the green tungsten carbide inserts prior to the heat treatment, called sintering, which consolidated the tungsten carbide for final use. This enabled more complex configurations to be formed in the face of the inserts.
An example of one of the early inserts is shown in the U.S. Pat. to Dowd, No. 3,137,917. More recent examples are illustrated in U.S. Pat. Nos. 3,792,515; 3,875,663; and 3,973,307. The latter patent shows a raised, peripheral land extending around the insert and a drop-off from this land to a flat and lower surface which occupies the central portion of the insert.
It will be appreciated that there are many variables in a cutting operation quite apart from the material being cut. These include the depth of cut, the rotational speed in lathe cutting, for example, and the feed rate.